Sustainable Production of Bioactive Molecules from C-Lignin-Derived Propenylcatechol

Catalytic Hydrogenolysis of Lignin into Serviceable Products

ConspectusLignin, a major component of lignocellulosic biomass, accounts for nearly 30% of organic carbon on Earth, making it the most abundant renewable source of aromatic carbon. The valorization of lignin beyond low-value heat and power has been one of the foremost challenges for a long time. On the other hand, aromatic compounds, constituting a substantial segment of the chemical industry and projected to reach a market value of $382 billion by 2030, are predominantly derived from fossil resources, contributing to increased CO2 emissions. Integrating lignin into the aromatic chemical supply chain will offer a promising strategy to reduce the carbon footprint and boost the economic viability of biorefineries. Thus, depolymerizing lignin biopolymers into aromatic chemicals suitable for downstream processing is an important starting point for its valorization. However, owing to lignin's complexity and heterogeneity, achieving efficient and selective depolymerization that yields desirable, isolable aromatic monomers remains a significant scientific challenge.The structure of lignins varies significantly in terms of subunits and linkages across plant species, leading to considerable differences in their reactivity, in the distribution of resulting monomers, and in their subsequent utilization. In this context, this Account highlights our recent studies on the catalytic hydrogenolysis of lignin into serviceable products for preparing valuable materials, fuels, and chemicals. First, we designed a series of catalytic systems for lignin hydrogenolysis specifically tailored to the structural features of lignin from wood, grass, and certain seed coats. To reduce reliance on expensive commercial catalysts like Pd/C, Ru/C, and Pt/C, we advanced heterogeneous metal catalysts by shifting from high-loaded nanostructured metals to low-loaded, atomically dispersed metals and replacing precious metals with nonprecious alternatives. This approach significantly reduces the cost of catalysts, enhances their atomic economy, and improves their catalytic activity and/or selectivity. Then, using the developed catalysts, phenolic monomers tethering a distinct side chain were selectively generated from the hydrogenolysis of lignin (from various plants), achieving yields close to the theoretical maximum. The high selectivity allowed the separation and purification of monomeric phenols from lignin reaction mixtures readily. To gain deeper insights into the cleavage of lignin C-O bonds, we designed deuterium-incorporated β-O-4 mimics (dimers and one polymer) for a mechanistic study, which excluded the pathways involving the loss of linkage protons and led to the proposal of a concerted hydrogenolysis process for β-O-4 cleavage. Finally, to enable the utilization of depolymerized lignin phenolic monomers, unconventional feedstocks in the current chemical industry, we developed a series of methods to transform them into valuable bioactive molecules, functional materials, and high-energy fuels. Overall, these contributions opened new avenues for converting lignin into serviceable products, encompassing upstream processing and downstream applications.

Chemical Synthesis of Monolignols: Traditional Methods, Recent Advances, and Future Challenges in Sustainable Processes

Monolignols represent pivotal alcohol-based constituents in lignin synthesis, playing indispensable roles in plant growth and development with profound implications for industries reliant on wood and paper. Monolignols and their derivates have multiple applications in several industries. Monolignols exhibit antioxidant activity due to their ability to donate hydrogen atoms or electrons to neutralize free radicals, thus preventing oxidative stress and damage to cells. Characterized by their alcohol functionalities, monolignols present three main forms: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. In nature, particularly in plants, monolignols with geometry (E) predominate over their Z counterparts. The methods for obtaining the three canonical monolignols, two less-common monolignols, and a monolignol analogue are addressed to present an overview of these phenol-based compounds, particularly from a synthetic standpoint. A SWOT (Strengths, Weaknesses, Opportunities, and Threats) analysis is used to explain the advantages and disadvantages of synthesizing monolignols, key alcohol-containing raw materials with enormous significance in both plant biology and industrial applications, using bench chemical methods. The uniqueness of this work is that it provides an overview of the synthetic pathways of monolignols to assist researchers in pharmaceutical and biological fields in selecting an appropriate procedure for the preparation of their lignin models. Moreover, we aim to inspire scientists, particularly chemists, to develop more sustainable synthetic protocols for monolignols.

Study on the efficient electrocatalytic depolymerization of α-O-4 bond in lignin assisted by simple electrolyte

Lignin is anticipated to serve as a replacement for dwindling fossil fuel resources owing to its abundant sources and renewable nature. The electrochemical oxidation technique for depolymerizing lignin has garnered significant interest for its environmentally friendly and mild operating conditions. Nevertheless, the current utilization of auxiliary electrolytes, predominantly organic bases, ionic liquids, and other specialized substances, poses a constraint on the widespread adoption of this approach. Furthermore, there is a scarcity of instances where electrochemical technology has been employed to depolymerize the α-O-4 bond in lignin for the production of highly selective acetals. In this study, a sodium chloride/methanol (NaCl/MeOH) system was utilized for the direct depolymerization of the α-O-4 bond in a lignin model molecule, specifically benzyl phenyl ether (BPE). The optimal conditions resulted in a 95.2 % conversion rate of the BPE substrate and a high yield of 94.5 % for the main product, benzaldehyde dimethyl acetal(Bda). This research offers a promising approach for the electrocatalytic depolymerization of α-O-4 bonds in lignin, leading to the selective production of acetal chemicals using a common auxiliary electrolyte at room temperature in just 2 h.

Benzenoid Aromatics from Renewable Resources

In this Review, all known chemical methods for the conversion of renewable resources into benzenoid aromatics are summarized. The raw materials that were taken into consideration are CO2; lignocellulose and its constituents cellulose, hemicellulose, and lignin; carbohydrates, mostly glucose, fructose, and xylose; chitin; fats and oils; terpenes; and materials that are easily obtained via fermentation, such as biogas, bioethanol, acetone, and many more. There are roughly two directions. One much used method is catalytic fast pyrolysis carried out at high temperatures (between 300 and 700 °C depending on the raw material), which leads to the formation of biochar; gases, such as CO, CO2, H2, and CH4; and an oil which is a mixture of hydrocarbons, mostly aromatics. The carbon selectivities of this method can be reasonably high when defined small molecules such as methanol or hexane are used but are rather low when highly oxygenated compounds such as lignocellulose are used. The other direction is largely based on the multistep conversion of platform chemicals obtained from lignocellulose, cellulose, or sugars and a limited number of fats and terpenes. Much research has focused on furan compounds such as furfural, 5-hydroxymethylfurfural, and 5-chloromethylfurfural. The conversion of lignocellulose to xylene via 5-chloromethylfurfural and dimethylfuran has led to the construction of two large-scale plants, one of which has been operational since 2023.

Advancement of lignin into bioactive compounds through selective organic synthesis methods

The conversion of lignin into bioactive compounds through selective organic synthesis methods represents a promising frontier in the pursuit of sustainable raw materials and green chemistry. This review explores the versatility of lignin-derived bioactive compounds, ranging from their application in drug discovery to their role in the development of biodegradable materials. Despite notable advancements, the synthesis routes and yields of highly bioactive molecules from lignin still require further exploration and improvement. This review provides an in-depth examination of the progress made in understanding the complex structure of lignin and developing innovative approaches to exploit its potential. Specifically, the types of lignins covered include softwood Kraft lignin, hardwood organosolv lignin, and soda lignin. This work is divided into three parts: first, the transformation of lignin into bioactive molecules with chemically active centres and functionalised hydroxyl groups through depolymerisation; second, kinetic modelling techniques essential for understanding the chemical kinetics of lignin and enabling significant scaling up in the conversion of organic molecules; third, efficient catalytic pathways for synthesising molecules with anticancer and antibacterial properties. In conclusion, this comprehensive review spurs further investigations into lignin-derived bioactive compounds, their applications, and the advancement of sustainable processes.

Advancements and Perspectives toward Lignin Valorization via O-Demethylation

Lignin represents the largest aromatic carbon resource in plants, holding significant promise as a renewable feedstock for bioaromatics and other cyclic hydrocarbons in the context of the circular bioeconomy. However, the methoxy groups of aryl methyl ethers, abundantly found in technical lignins and lignin-derived chemicals, limit their pertinent chemical reactivity and broader applicability. Unlocking the phenolic hydroxyl functionality through O-demethylation (ODM) has emerged as a valuable approach to mitigate this need and enables further applications. In this review, we provide a comprehensive summary of the progress in the valorization of technical lignin and lignin-derived chemicals via ODM, both catalytic and non-catalytic reactions. Furthermore, a detailed analysis of the properties and potential applications of the O-demethylated products is presented, accompanied by a systematic overview of available ODM reactions. This review primarily focuses on enhancing the phenolic hydroxyl content in lignin-derived species through ODM, showcasing its potential in the catalytic funneling of lignin and value-added applications. A comprehensive synopsis and future outlook are included in the concluding section of this review.

Sustainable production of catechol derivatives from waste tung nutshell C/G-type lignin via heterogeneous Cu-NC catalytic oxidation

The sustainable production of catechol derivatives is a challenging task. Catechyl (C) and guaiacyl (G) lignins coexisting in waste tung nutshells are promising feedstocks to form valuable catechol derivatives, but the depolymerization of C/G lignin typically involves a catalytic reductive process that cannot produce these oxidized aromatic chemicals. Herein, we demonstrated that the sustainable production of catechol derivative aldehydes and acids from C/G lignin could be achieved through a heterogeneous copper-catalyzed oxidative process. Under optimized conditions, the Cu-NC-800 catalyst affords a 43.5 mg g-1 yield (8.9 wt%, based on Klason lignin) of aromatic aldehydes (protocatechuic aldehyde, vanillin) and acids (protocatechuic acid, vanillic acid). XRD and XPS analyses showed that CuO and Cu2O may be the active species during the heterogeneous oxidation of the Cu-NC-800 catalyst. This study opens new opportunities for the sustainable production of catechol derivatives from C/G-type lignin.